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Watching an episode of Alton Brown's "Good Eats" (episode: House of the Rising Bun) and AB is demonstrating a recipe for cinnamon rolls. In the "wet" mix he includes both buttermilk and butter.

Now, since buttermilk is milk after the butter has been churned out, why would a recipe call for both?

Obviously, proportion would be an issue, but whole milk + (less) butter should equal buttermilk + (full amount) butter, shouldn't it?

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@Aaronut: fundamentally this question is about the choice of Buttermilk + Butter vs. Milk + Butter as an "ingredient selection" –  Cos Callis Oct 11 '11 at 0:58
    
One of those tags does not apply and the other should never be used, period. Please read the tag excerpts and wikis on the ingredient tags for more information. –  Aaronut Oct 11 '11 at 2:05
    
@Aaronut, if you are going to 'remove a tag' remove it, there is no legacy value in allowing it to linger on out there were it can be found. Particularly one with a prima-facia value that should not require the 'reading of the wiki'. As for "ingredient selection", if this isn't included in the meaning of that tag, it should be. What tag would you recommend for "what reason is there to use X over Y" –  Cos Callis Oct 11 '11 at 2:22
    
@CosCallis I had to learn that one the hard way too, it's rather confusing. "ingredient-selection" would be things like "How do I choose an apple that's ripe", not "why did the recipe author select to use apples in this recipe". –  Yamikuronue Aug 31 '12 at 14:06
    
Buttermilk is not "milk after the butter has been churned out." Traditional buttermilk is fermented (soured, not sour) cream after the butter has been churned out. If you happen to get your hands on raw milk, skim the cream, and let it ferment for a few days, maybe you could substitute it for traditional buttermilk + butter. But most recipes these days assume cultured buttermilk anyway, which is slightly different and has absolutely nothing to do with butter manufacturing. The assumptions in the question are wrong, so the question does not make sense. –  Athanasius Jan 9 '13 at 3:10
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3 Answers

up vote 12 down vote accepted

Hah, I get to cite my new copy of McGee's "On Food and Cooking" for the first time! There are several things going on here (all of which can be found in the 2004 edition of McGee, most on page 50).

Firstly, as Nathan indicates in his answer, most of the liquid that is sold as buttermilk these days is in fact not "real" buttermilk, but so-called "cultured buttermilk", made from ordinary skim milk and fermented until acid and thick.

Secondly, even traditional buttermilk was somewhat sour, though less than cultured buttermilk. To find out why, let's examine the processes that were involved in making it. If you want to make butter, you start by separating milk into cream and whey. Since the 19th century, we typically do that with a centrifuge and it goes quite quickly, but before then, we would leave it to gravity. That would take a while, and the milk would start to ferment while it happened - especially in warmer environments. This fermenting is essentially lactic acid bacteria converting lactose into lactic acid, which sours the milk a little.

So in either case, you would have more lactic acid in butter + buttermilk than in butter + whole milk.

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Great breakdown. More to buttermilk than meets the eye there is. –  Cos Callis Oct 11 '11 at 2:18
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Two things:

First, commercially available buttermilk is actually milk that has been slightly cultured to increase the amount of lactic acid; that's why it tastes tart. My grandmother used to make 'buttermilk' by leaving some milk on the counter to let it ferment. Because of the increased amount of acid, a lot of recipes (like buns or biscuits) mix buttermilk with baking powder to leaven the mix.

Secondly, the order that they are added might matter. I'm not familiar with the recipe, but he might want to add the butter at an earlier or latter time. He might do the former to cream the butter (mix butter and sugar at a high speed to create bubbles of air in the butter; it's what you do with cake), or he might fold the butter into the mix later on to help create flakiness.

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The ingredients are added together in the 'wet mix', but still your first point is interesting and your second could well apply in other situations. Thanks for your input. –  Cos Callis Oct 11 '11 at 2:17
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The second point is the main one, I think. Baking is about texture and the distribution of air and hydration. Fat emulsified in water (=milk) has a very different effect from a blob of fat (=butter) together with some water. Saying that buttermilk+butter is the same as milk is like saying that a pie shell is the same as a roux sauce, because both is butter + flour plus some liquid. –  rumtscho Oct 11 '11 at 9:39
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The need to mix buttermilk with butter comes from needing a substitute for sour cream in baking since some people cannot use milk, being lactose intolerant. See "The Milk Sugar Dilemma: Living with Lactose Intolerance", substitutions, page 120, Second Edition, Revised and Expanded by Richard A. Martens, M.D. and Sherlyn Martens, M.S., R.D.

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